BAGHDAD, Iraq -- Saddam Hussein's government is believed to have buried as many as 300,000 opponents in 263 mass graves that dot the Iraqi landscape, the top human rights official in the U.S.-led civilian administration said Saturday.

Sandy Hodgkinson said the administration has been sending forensic teams to investigate those grave sites reported to U.S. officials. So far, the existence of about 40 graves has been confirmed.

"We have found mass graves with women and children with bullet holes in their heads," she said.

President Bush has referred to Iraqi mass graves frequently in recent months, saying they provide evidence that the war to drive Saddam from power was justified.

But some human rights activists have criticized the U.S.-led administration in Iraq for moving too slowly to protect grave sites and begin excavations, and have expressed skepticism that it will ever fully identify who is buried in the mass graves.

"There is just no way -- technologically, financially -- that they're going to deal with mass graves on this magnitude," said Susannah Sirkin of Physicians for Human Rights in Boston.

The U.S.-led administration held a workshop Saturday to train dozens of Iraqis to find and protect the mass grave sites. Hodgkinson said the workers would be crucial in protecting the sites from desperate relatives trying to dig for evidence of their missing loved ones.

In the weeks after the U.S.-led war drove Saddam from power, relatives damaged some grave sites, using bulldozers that mangled bodies and scattering papers and clothing that could have been used to identify remains.

The largest mass grave discovered so far, a site near the southern town of Mahaweel believed to hold at least 3,115 bodies, was damaged by relatives searching for remains. But officials say most of the mass graves haven't been disturbed.

Mass graves "tell the story of missing loved ones such as where, when and how they were killed," Hodgkinson said. "Truth and proper burial is the first step toward reconciliation."

Iraqi Human Rights Minister Abdul-Basit Turki said that in addition to families' need to find the bodies of missing relatives, excavating mass graves is important in building criminal cases against members of the former regime.

International tribunals handle prosecutions for atrocities in the former Yugoslavia, where tens of thousands of missing are believed buried in mass graves, and Rwanda, in which many of the 500,000 victims of a 100-day killing spree in 1994 were buried in communal pits.

But for Iraq, the United States has insisted any trials be conducted by a new Iraqi legal system that is still being developed.

Neither Iraq nor the United States are signatories to the International Criminal Court and it would take a vote of the U.N. Security Council to create a special tribunal for Iraq, which is considered unlikely.

Many human rights groups agree that Iraqis should lead the legal process, but say international participation is crucial for it to be legitimate and impartial. Some have been hesitant to participate in excavations before the legal system is in place.

"Mass graves really can corroborate witness testimony and documents which show what happened in a crime," Hodgkinson said, although she cautioned: "a mass grave by itself won't tell you who did it."

Hodgkinson said the majority of people buried in the mass graves are believed to be Kurds killed by Saddam in the 1980s after rebelling against the government and Shiites killed after an uprising following the 1991 Gulf War.

Hodgkinson said the investigation process would be similar to that used in Bosnia after its 1992-95 war. But she cautioned that if Bosnia is any indication, the process in Iraq will be long and complicated.

In Bosnia, she said, it has taken nine years to unearth 8,000 of the 30,000 bodies believed buried in mass graves.

Human rights activists say U.S. authorities in Iraq have been much slower to address the problem than were authorities in Bosnia. In Bosnia, said Sam Zia-Zarifi of Human Rights Watch, "within the first year there were 25 teams in and a (U.N.) tribunal in place."

In Iraq, some international teams that were hoping to begin their work before winter have delayed their arrival because of violence, including the bombing of the U.N. headquarters in Baghdad.

At a donor conference last month, more than $100 million was requested for uncovering mass graves. The donations, which are expected to come in the form of equipment and personnel, would be used over five years, Hodgkinson said.